The present invention relates generally to a system for analyzing the frequency components of an input signal, and more particularly relates to a new and improved sweeping spectrum analyzer which utilizes optical techniques for processing electrical signals. Recent advances in optical technology have made optical processing of electrical signals far simpler and less expensive for certain frequency analyzer applications than conventional electronic processing.
More particularly, the spectrum analyzer of the present invention utilizes a Bragg cell in combination with a charge coupled photodetector array to achieve both high resolution and high sensitivity. A Bragg cell is usually a block of glass or crystalline material approximately 1 cm. .times. 1 cm. in cross section and up to 10 to 20 centimeters long. A piezoelectric transducer is bonded to the end of the cell and turned to the frequency band of interest. When the transducer is excited with an electrical signal, a travelling acoustic wave is set up in the cell. This causes slight changes in the refractive index of the cell between the peaks and valleys of the acoustic pressure wave. When light is introduced at the correct angle, termed the Bragg angle, the refractions from the index changes add in phase, and Bragg diffraction takes place. A portion of the input light beam is deflected, and can be imaged onto a screen or photodetector. The amplitude of the deflected beam is proportional to the amplitude of the acoustic input and the deflection angle is proportional to the frequency of the acoustic input. If a radio electric signal is fed into the Bragg cell, a spot of light is imaged the position of which is proportional to the signal frequency and the amplitude of which is proportional to the instantaneous signal strength. Thus, all of the modulation on the signal is preserved. If there are simultaneous multiple input signals at different frequencies within the Bragg cell bandwidth, they will be simultaneously imaged at different positions in the Bragg cell output. This is one of the key advantages of Bragg cell signal processing. Multiple signals can be processed simultaneously without the necessity for time sharing or sweeping. Another advantage is that the output is in a form suitable for further signal processing with recently developed complex detector arrays.
With Bragg cells a wide variety of operating parameters can be achieved. The efficiency of deflection is very high, and most applications require less than a watt of drive power. When there is no input there is no deflection and hence there is no output. Thus, the extinction ratio is essentially infinite.